UCSF

A lingering question in developmental biology has centered on how PBX TALE transcription factors, viewed primarily as HOX co-factors, can confer functional specificity to spatially-restricted HOX proteins despite being ubiquitously distributed in vertebrate embryos. Here, using the murine hindlimb as a model, we investigate the elusive mechanisms whereby PBX homeoproteins themselves attain tissue-specific developmental functions. We first demonstrate that mesenchymal-specific loss of PBX1/2 or the transcriptional regulator HAND2 generates similar limb phenotypes. By combining tissue-specific and temporally-controlled mutagenesis with multi-omics approaches, we then reconstruct a GRN at organismal-level resolution that is collaboratively directed by PBX1/2 and HAND2 within subsets of posterior-proximal hindlimb mesenchymal cells. Genome-wide profiling of PBX1 binding across multiple embryonic tissues further reveals that HAND2 selects a subset of PBX-bound regions to impart limb patterning functionality. Our research elucidates fundamental principles by which promiscuous transcription factors cooperate with select cofactors to instruct distinct developmental programs.